Process for barrier coating of plastic objects

ABSTRACT

A method for sequentially depositing a silicon oxide based film as a barrier on a substrate. The film is useful for providing an effective barrier against gas permeability in containers and for extending shelf-life of containers, especially plastic evacuated blood collection devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for depositing a barrier coating toplastic containers for improving the effective barrier of the plasticcontainer against gas and water permeability.

2. Description of the Related Art

In numerous fields of technology, it is necessary to apply very thincoatings of pure substances to certain objects. An example is windowglass which is provided with a thin coating of metal oxide in order tofilter certain wavelength ranges out of sunlight. In semi-conductortechnology, thin coatings of one or more substances are often applied toa substrate. It is especially important that the thin coatings not onlybe pure, but also that they be precisely measured out so that thecoating thickness and, in the case of coatings of chemical compounds,their composition will be accurately repeatable. These coatingthicknesses are, as a rule, between two and several thousands ofnanometers.

A variety of methods are known for applying thin coatings to films,glass and other substrates. Such processes for depositing SiO_(x) ontoplastic objects are disclosed in U.S. Pat. No. 5,053,244 and EuropeanPatent No. 0 299 754. Most specifically, these processes can provideexcellent barrier properties to plastic films which have not beenexposed to dust. However, these processes provide only minor barrierproperties to three-dimensional plastic objects or films that areexposed to dust.

It is believed that the inability to obtain a good barrier onthree-dimensional plastic objects is due to the lack of cleanliness ofthe object's surface since most three dimension objects are exposed todust during fabrication.

In the process of depositing SiO_(x) on thin films, the SiO_(x) barriercoating is applied under vacuum conditions to a clean film. Typically,film is extruded under very clean conditions and immediately wound intoa large roll. As a consequence, the film surfaces, with the exception ofthe outside layer, are never exposed to particles in air such as dust.

It is believed that the reason for the lack or minimum improvement inpermeability of three-dimensional objects coated with SiO_(x) is thatthe surface of the three-dimensional object has a contaminated surface.It is further believed that the contamination is due to foreign surfaceparticles that settle on the object due to its exposure to air.

Even though SiO_(x) is evenly deposited on the surface of an object atabout 500 to 1000 Å in thickness, because foreign surface particles,that are on the order of 5000 to 50000 Å in diameter, may be on thesurface of the object, portions of the surface are not coated with theSiO_(x) because of the shielding effect caused by the foreign surfaceparticles.

Therefore, a need exists to remove and/or redistribute contaminationfrom the surface of objects that are to be coated with SiO_(x) toimprove the process for applying SiO_(x) to the objects and moreparticularly, to improve the barrier properties of the objects.

SUMMARY OF THE INVENTION

The present invention is a process for sequentially depositing a barriercomposition over the outer surface of an article, such as a compositecontainer.

Preferably, the barrier composition is a silicon oxide based film. Sucha film desirably is derived from volatile organosilicon compounds orgases.

Most preferably, the method for sequentially depositing a silicon oxidebased film on an article, such as a plastic collection tube comprisesthe following steps:

(a) controllably flowing a gas stream including an organosiliconcompound into a plasma;

(b) depositing a first silicon oxide based film onto the surface of thearticle while maintaining a pressure of less than about 100 microns Hgduring the depositing;

(c) removing and/or redistributing foreign surface particles from thesurface of the article; and

(d) depositing a second silicon oxide based film onto the surface of thearticle over the first silicon based film by repeating steps (a) through(b) above.

The organosilicon compound is preferably combined with oxygen and heliumand at least a portion of the plasma is preferably magnetically confinedadjacent to the article during the depositing, most preferably by anunbalanced magnetron.

Preferably, the foreign surface particles are removed and/orredistributed from the surface of the article by ultrasonic vibrationsor wiping. Most preferably the foreign surface particles are removedand/or redistributed with pressurized gas.

Preferably, the film provides a transparent, translucent or colorlessappearance and may have printed matter applied thereon.

Alternatively, the process of depositing a silicon oxide based film canbe accomplished by evaporating or sputtering a metal or metal oxideinstead of plasma.

The advantage achieved by the process of sequentially depositing asilicon oxide based film with the intermittent rein oval and/orredistribution of foreign surface particles from the surface of thearticle is that improved permeability is observed.

A further advantage is that the process of the present inventionimproves the permeability of three-dimensional objects that has not beenachieved with conventional deposition processes typically used with thinfilms.

A significant advantage of the process of the present invention is thatthe intermittent step of removing and/or redistributing foreign surfaceparticles on the surface of the article exposes the regions shielded bythe foreign surface particles for the depositing of a second SiO_(x)coating. Therefore, a significant reduction in permeability of thearticle is due to the complete SiO_(x) surface coverage that is obtainedby the sequential deposition technique and intermittent step of removingand/or redistributing the foreign surface particles.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic diagram illustrating the sequentialdepositing of SiO_(x).

FIG. 2 is a general schematic diagram illustrating a plasma systemutilizing the various aspects of the present invention.

FIG. 3 is a schematic diagram illustrating a side sectional view of theplasma deposition chamber and its associated equipment as related toFIG. 2.

FIG. 4 illustrates the use of a balanced magnetron in the system of FIG.3.

FIG. 5 illustrates the use of an unbalanced magnetron in the system ofFIG. 3.

DETAILED DESCRIPTION

The present invention may be embodied in other specific forms and is notlimited to any specific embodiment described in detail which is merelyexemplary. Various other modifications will be apparent to and readilymade by those skilled in the art without departing from the scope andspirit of the invention. The scope of the invention will be measured bythe appended claims and their equivalents.

The method for sequentially depositing a silicon oxide based film on asubstrate is preferably conducted in a previously evacuated chamber ofglow discharge from a gas stream. The gas stream preferably comprises atleast three components: a volatilized organosilicon component, anoxidizer component such as oxygen and an inert gas component.

Suitable organosilicon compounds for the gas stream are liquid at aboutambient temperature and when volatilized have a boiling point aboveabout ambient temperature and include methylsilane, dimethysilane,trimethylsilane, diethylsilane, propylsilane, phenylsilane,hexamethyldisilane, 1,1,2,2 -tetramethyl disilane, his (trimethylsilane) methane, bis (dimethylsilyl) methane, hexamethyldisiloxane,vinyl trimethoxy silane, vinyl triethyoxysilane, ethylmethoxy silane,ethyltri methoxy silane, divinyltetramethyldisiloxane,divinylhexamethyltrisfloxane, and trivinylpentamethyltrisfloxazane.These preferred organosilicon compounds have boiling points of 71° C.,102° C., 123° C. and 127° C. respectively.

Among the preferred organosilicons are 1,1,3,3-tetramethyldisiloxane,hexamethyldisiloxane, vinyltrimethylsilane, methyltrimethoxysilane,vinyltrimethoxysilane and hexamethyldisilazane. These preferredorganosilicon compounds have boiling points of 71° C., 101° C., 55.5°C., 102° C., 123° C. and 127° C. respectively.

The inert gas of the gas stream preferably is helium or argon. Mostpreferably the inert gas is helium.

The volatilized organosilicon component is preferably admixed with theoxygen component and the inert gas component before being flowed intothe chamber. The quantities of these gases being so admixed arecontrolled by flow controllers so as to adjustably control the flow rateratio of the gas stream components.

The organosilicon compound and oxygen of the gas stream during thedepositing are preferably in a flow rate ratio between about 1.2:1 toabout 1:1.8. When the inert gas is helium or argon, then the preferredflow rate ratio of organosilicon compound, oxygen and inert gas is about1 to 1.8:1.5 to 1.8 to 2.3.

In addition to the necessary organosilicon, oxygen and inert gas in thegas stream, minor amounts (not greater than about 1:1 with respect tothe organosilicon, more preferably about 0.4 to 1.1:1 with respect tothe organosilicon) of one or more additional compounds in gaseous formmay be included for particular desired properties. These additionalcompounds include, but are not limited to a lower hydrocarbon such aspropylene, methane or acetylene, or nitrogen. In particular, nitrogenincreases the deposition rate, improves the transmission and reflectionoptical properties on glass, and varies the index of refraction inresponse to varied amounts of N₂. The addition of nitrous oxide to thegas stream increases the deposition rate and improves the opticalproperties, but tends to decrease the film hardness.

A particularly preferred gas stream composition has 20 to 40 SCCMorganosilicon, 20 to 40 SCCM 0₂, 40 to 60 SCCM He, 1 to 10 SCCMpropylene and 5 to 20 SCCM N₂.

A glow discharge plasma is established in the previously evacuatedchamber which is derived from one or more of the gas stream components,and preferably is derived from the gas stream itself. The article ispositioned in the plasma, preferably adjacent the confined plasma, andthe gas stream is controllably flowed into the plasma. A first siliconoxide based film is deposited on the substrate.

As shown in FIG. 1, the first silicon oxide based film A does notcompletely cover surface B of substrate C. It is believed that completecoverage cannot be achieved with the first silicon oxide based filmbecause of surface shielding caused by foreign surface particles D. Ineffect, no silicon oxide based film is applied beneath the particles.

Therefore, before the second silicon oxide based film is applied, theshielding particles are removed and/or redistributed from the surface ofthe substrate with compressed gas. The compressed gas is most preferablyair, nitrogen, argon or oxygen. The pressure of the gas is preferably atabout 25 psi. Shielded regions E are exposed after the foreign surfaceparticles are removed and/or redistributed with the compressed gas.

After the foreign surface particles are removed and/or redistributedwith the compressed gas, the substrate is then again subjected to theglow discharge plasma process described above to deposit a secondsilicon based oxide film F and to produce a substantially pin hole freebarrier coating on the substrate.

The deposition method of the present invention is preferably practicedat relatively high power and quite low pressure. A pressure less thanabout 100 microns Hg (0.1 Tort) should be maintained during thedeposition, and preferably the chamber is at a pressure between about 43to about 49 microns Hg during the deposition of film.

The substrate is electrically isolated from the deposition system(except for electrical contact with the plasma) and is at a temperatureof less than about 80° C. during the depositing. That is, the substrateis not deliberately heated.

Referring to FIG. 2, the system for sequentially depositing a siliconoxide based film and removing and/or redistributing surface particles isschematically illustrated. The system for depositing a silicon oxidebased film comprises an enclosed reaction chamber 11 in which a plasmais formed and in which a substrate 13, is placed for depositing a thinfilm of material on it. The substrate can be any vacuum compatiblematerial, such as plastic. One or more gases are supplied to thereaction chamber by a gas supply system 115. An electric field iscreated by a power supply 17, and a low pressure is maintained by apressure control system 19. An optical emission spectrometer 21 isconnected through an optical fiber light transmission medium 23 to thereaction chamber in some appropriate manner to couple the visible andnear visible (especially the ultraviolet range) emission of the plasmato the spectrometer. A quartz window 24 in a side wall of the reactionchamber can be used to optically couple the plasma emission withexternal fiber medium 23. A general control system 25, including acomputer control portion, is connected to each of the other componentsof the system in a manner to receive status information from and sendcontrolling commands to them.

The reaction chamber can be of an appropriate type to perform any of theplasma-enhanced chemical vapor deposition (PECVD) or plasmapolymerization process.

The system for removing and/or redistributing foreign surface particlesincludes an enclosed chamber 10 with a compressed gas supply system 20for removing and/or redistributing the foreign surface particles thatmay be on the substrate.

FIG. 3 shows a detailed side sectional schematic of the system of FIG.2. In particular, reaction chamber 11 is divided into a load lockcompartment 27 and a process compartment 29 by an isolation gate valve31. Pressure control system 19 includes a mechanical pump 33 connectedto load lock compartment 27 by a valve 315. The pressure control systemalso includes diffusion pumps 37 and 39, and an associated mechanicalpump 41. Diffusion pump 37 is connected to load lock compartment 27through an isolation gate valve 43 and an adjustable baffle 415.Similarly, diffusion pump 39 is connected to process compartment 29through an isolation gate valve 47 and an adjustable baffle 49. Baffle49 is controlled by system control 25, while a coating process is beingcarried out, in order to maintain the internal pressure at a desiredvalue.

The substrate to be coated is first loaded into load lock compartment 27with valve 31 closed. Mechanical pump 33 then reduces the pressure mostof the way to the high vacuum region. Diffusion pump 37 is then operatedto reduce the pressure further, to about 5×10⁻⁶ Torr. The operatingpressure is about 46 microns for a PECVD or plasma polymerizationprocess and is achieved by flowing the process gases into the reactionchamber and throttling diffusion pump 39 using baffle 49. During loadingand unloading operations, diffusion pump 39 maintains the depositionchamber 29 at the operating pressure. Once load lock compartment 27 isreduced to base pressure, valve 31 is opened and substrate 13 is movedinto process compartment 29.

During the deposition process, substrate 13 is moved back and forththrough a plasma region 51, a number of times in order that the thinfilm deposited on the outer surface of the substrate has a desireduniform thickness.

A magnetron is positioned within chamber 29, formed of a magneticstructure 55 and a cathode 57. Power supply 17 has its output connectedbetween cathode 57 and a metallic body of the reaction chamber. Themagnetron creates an appropriate combination of magnetic and electricalfields in plasma region 51 in order to create a plasma there when theproper gases are introduced into the process compartment. The substrateis maintained electrically isolated and is passed directly through theplasma region.

The gaseous components necessary for the plasma to form in plasma region51 are introduced into deposition chamber 29 by a conduit 59. Gas flowswithin deposition chamber 29 from diffusion pump 39. A pair of baffles61 and 63 on either side of the magnetron help to confine the gas flowto plasma region 51.

After a first coating of the silicon oxide based film has been depositedon the substrate, the substrate is subjected to a stream of compressedgas at about 25 psi in chamber 10. Foreign surface particles on thesubstrate are removed and/or redistributed. The substrate is thenreturned to chamber 11 for a second coating of the silicon oxide basedfilm.

The magnetron used in process compartment 29 can be of a planarmagnetron form as shown in FIG. 4. A cross-sectional view of the magnetstructure 55 is provided at a vertical plane. In plan view, thestructure of FIG. 4 is elongated in a direction normal to the plane ofpaper.

The structure of FIG. 4 is termed a balanced magnetron. Its magneticlines of force 131 travel between one of the outer south magnetic polesand a central north pole. As is well known, electrons and ions travel ina spiral around a magnetic force line and along it, under influence of acombination of the magnetic field forces and the electric field forcesformed by the cathode and the process chamber metal case. Cathode 57 isgenerally made of titanium or quartz, but sputtering is prevented fromhappening because of the higher pressure used in the deposition systemof FIG. 3.

An unbalanced magnetron that alternatively can be utilized in the systemof FIG. 3 is shown in FIG. 5. Outside magnets 133 and 135 are arrangedwith a soft iron core 137 middle. Only the south magnetic poles arepositioned against a cathode 57, the north pole faces being orientedaway from the cathode. The result is that a substantial proportion ofthe magnetic field line follow a much longer path in extending betweenthe magnetic: south and north pole regions.

The magnetron structures of FIGS. 4 and 5 are suitable for low frequencyoperation of power supply 17. An example frequency is 40 kHz. However,there can be some advantages from operating at a much high frequency,such as in the radio frequency range of several megahertz.

The silicon oxide based film or blends thereof used in accordance withthis disclosure, may contain conventional additives and ingredientswhich do not adversely affect the properties of articles made therefrom.

Various other modifications will be apparent to and may be readily madeby those skilled in the art without departing from the scope and spiritof the invention.

A variety of substrates can be coated with a barrier composition by theprocess of the present invention. Such substrates include, but are notlimited to packaging, containers, bottles, jars, tubes and medicaldevices.

A variety of processes are also available in addition to plasmadeposition for depositing a barrier composition. Such processes include,but are not limited to radio frequency discharge, direct or dual ionbeam deposition, or sputtering.

The following exam pies are not limited to any specific embodiment ofthe invention, but are only exemplary.

EXAMPLE 1 METHOD FOR COATING A PLASTIC BLOOD COLLECTION TUBE WITH ASILICON OXIDE BASED FILM

An enclosed reaction chamber was evacuated to a base pressure of notgreater than about 3×10⁻⁶ Torr. The load lock was vented to atmospherewhile the chamber was maintained under high vacuum. Then the load lockwas evacuated with the plastic tube loaded therein. The chamber pressurewas adjusted to a desired value by adjusting the baffle over thediffusion pump. The load lock diffusion pump was closed and the valveisolating the load lock and the chamber was opened. After the pressurein the chamber was stabilized, the power supply was turned on andadjusted to the desired value and a glow discharge plasma wasestablished in the chamber. An emission spectrum from the controlprogram was used to find the appropriate oxygen to inert gas ratio. Theorganosilicon flow into the chamber was then adjusted until the desiredoxygen to inert gas ratio was obtained. The tube was then conveyed backand forth through the plasma region until the first coating thicknesswas achieved while continuing to monitor the process conditions. Oncethe first desired film thickness was obtained, the system was shut downand the coated tube was removed. The coated substrate was then placed ina second chamber with a compressed gas supply. The substrate was thensubjected to the compressed gas to remove or redistribute the foreignsurface particles. The substrate was then returned to the reactionchamber for a second coating of silicon oxide.

EXAMPLE 2 COMPARISON OF FILMS COATED AND UNCOATED WITH A SILICON OXIDEBASED FILM

The permeability of flat substrates made of polyethyleneterephthalate(PET), 1 mil in thickness with and without a silicon oxide basedcoatings were measured at 25° C.

The results of the oxygen transmission rates are given below:

    ______________________________________                                                                        Oxygen                                                              Film      Flux                                                                Thickness (cm.sup.3 /m.sup.2                            SAMPLE                (Å)   day atm)                                      ______________________________________                                        Uncoated PET film (1 mil)                                                                            0        80                                            SiO.sub.x on PET film without                                                                       500       35                                            particle removal (1 deposition)                                               SIO.sub.x on PET film; vent chamber                                                                 500       22                                            to atmosphere between depositions                                             SIO.sub.x on PET film; spray surfaces                                                               500       21                                            with carbon dioxide between depositions                                       SIO.sub.x on PET film; wipe surface                                                                 500       7.5                                           with lint-free tissue between depositions                                     SIO.sub.x on PET film; wipe surface                                                                 500       7.1                                           with a soft foam between depositions                                          SIO.sub.x on PET film; spray surface                                                                500       4.6                                           with helium between depositions                                               SIO.sub.x on PET film; spray surface with                                                           500       2.0                                           argon between depositions,                                                    Same film after irradiation (10.5 Mrads)                                                                      0.5                                           SiOx on PET film (AF power); spray                                                                  1750      0.5                                           surface with argon between depositions                                        ______________________________________                                    

All system parameters were kept essentially constant with the exceptionof the power source. A radio frequency (RF) power supply was used unlessindicated differently.

The results show that a lower transmission rate is achieved bysequentially depositing SiO_(x) with the intermittent removal and/orredistribution of surface particles with compressed gas, such as argon.

EXAMPLE 3 PROCEDURE FOR SEQUENTIAL PLASMA POLYMERIZATION OFTRIMETHYLSILANE/OXYGEN

A polymer substrate was cleaned in equal parts of a micro detergent andde-ionized (DI) water. The substrate was rinsed thoroughly in DI waterand allowed to air dry. The cleaned substrate was then stored in avacuum oven at room temperature until it was to be coated.

The cleaned substrate was then attached to a holder which fits midwaybetween the electrodes in the glass vacuum chamber. The chamber wasclosed and a mechanical pump was used to achieve a base pressure of 5mTorr.

The electrode configuration is internally capacitively coupled withpermanent magnets on the backside of the titanium electrodes. Thespecial configuration provides the ability to confine the glow betweenthe electrodes because of the increase in collision probability betweenelectrons and reacting gas molecules. The net result of applying amagnetic field is similar to increasing the power applied to theelectrodes, but without the disadvantages of higher bombardment energiesand increased substrate heating. The use of magnetron discharge allowsoperation in the low pressure region and a substantial increase inpolymer deposition rate.

The monomer which consists of a mixture of trim ethylsilane (TMS) andoxygen was introduced through stainless steel tubing near theelectrodes. The gases were mixed in the monomer inlet line beforeintroduction into the chamber. Flow rates were manually controlled bystainless steel metering valves. A power supply operating at an audiofrequency of 40 kHz was used to supply power to the electrodes. Thesystem parameters used for thin film deposition of plasma polymerizedTMS/O₂ on the polymer substrate were as follows:

TMS Flow=0.75-1.0 sccm

Oxygen Flow=2.5=3.0 sccm

System Pressure=90-100 mTorr

Power=30 watts

Deposition Time=5 minutes

After the thin film was deposited, the reactor was allowed to cool. Thereactor was then opened, and the sample holder is removed. The surfaceof the substrate was sprayed with a pressurized argon at approximately25 psi. The mechanical action removes or redistributes the particlesthat may interfere with uniform film coverage. The sample was thenimmediately returned to the vacuum chamber. The chamber pressure wasagain reduced to approximately 5 mTorr and the deposition processdescribed above was repeated.

What is claimed is:
 1. A method of sequentially depositing a siliconoxide based film on a plastic substrate in a previously evacuatedchamber by glow discharge comprising:(a) vaporizing an organosiliconcomponent and admixing the volatilized organosilicon component with anoxidizer component and an inert gas component to form a gas streamexterior the chamber; (b) controllably flowing the gas stream into saidchamber; (c) establishing a glow discharge plasma in the chamber fromsaid gas stream; (d) controllably flowing the gas stream into the plasmawhile confining at least a portion of the plasma therein; (e) depositinga first coating of silicon oxide on said substrate; (f) removing and/orredistributing foreign surface particles from said substrate; and (g)repeating steps (a) through (d) above, thereby depositing a secondcoating of a silicon oxide on said substrate.
 2. The method of claim 1,wherein said oxidizer component is oxygen.
 3. The method of claim 2,wherein the plastic substrate is electrically isolated from the chamberexcept for contact with the confined plasma.
 4. The method of claim 3,further comprising magnetically confining at least a portion of theplasma adjacent to the substrate during the depositing.
 5. The method ofclaim 4, wherein the organosilicon compound and oxygen of the gas streambeing flowed into the plasma are in a flow rate ratio between about1.2:1 to about 1:1.8 and the inert gas of the gas stream being flowedinto the plasma is helium or argon in an amount effective to increasethe deposition rate and the hardness of the deposited silicon oxide. 6.The method of claim 5, wherein the inert gas is helium in a ratio in therange from about 1:1.5 to 1:2.3.
 7. The method of claim 1, wherein theorganosilicon compound is 1,1,3,3-tetramethyldisiloxane,hexamethyldiallane, vinyltrimethylsilane, methyltri methoxyallane,vinyltrimethoxyallane or hexamethyldiallane or trimethylsilane.
 8. Themethod of claim 1, wherein said plastic substrate is conveyed into andout of the plasma during the depositing.
 9. The method of claim 1,wherein the inert gas is helium, the gas stream includes a minor amountof propylene, and the deposited silicon oxide based film includes carbonmoieties.
 10. The method of claim 1, wherein the gas stream includes aminor amount of nitrogen or nitrous oxide and the deposited siliconoxide based film includes nitrogen moieties.
 11. The method according toclaim 1, wherein said foreign surface particles are removed and/orredistributed with a pressurized gas stream.
 12. The method of claim 1,wherein said foreign surface particles are removed and/or redistributedby wiping said substrate.
 13. The method of claim 1, wherein saidforeign surface particles are removed and/or redistributed withultrasonic vibrations.
 14. The method of claim 11 wherein saidpressurized gas stream of step (e) is nitrogen or argon at about 25 psi.15. A method of depositing a film on a plastic substrate by a plasmaprocess within an evacuated chamber, comprising the steps of:(a)providing an evacuated chamber with a gas stream that includes a sourceof a material desired to be deposited on said substrate: (b)establishing within said chamber a glow discharge plasma derived fromthe gas of said stream in a region of high electric field; (c) removablypositioning tile plastic substrate in said plasma without any electricalconnection therewith; (d) generating within said plasma a magnetic fieldhaving a substantial magnetic flux directed against said substrate,thereby depositing a first coating of a silicon oxide based film on saidsubstrate; (e) removing and/or redistributing foreign surface particlesfrom said substrate; and (f) repeating steps (a) through (d) above,thereby depositing a second coating of a silicon oxide based film onsaid substrate.
 16. The method of claim 15, wherein the step ofgenerating a magnetic flux includes positioning within said chamber afirst magnetic pole oriented to face said plasma and a second magneticpole oriented to face away from said plasma.
 17. The method of claim 15,wherein the step of generating a magnetic flux includes positioningwithin said chamber a magnetic structure having a surface adjacent saidplasma that is characterized by a magnetic flux distribution function insubstantially any direction there across which varies from one magneticstrength of one polarity separated by a lesser magnetic strength ofanother polarity.
 18. The method of claim 15, wherein the plasma isconfined by means of an unbalanced magnetron.
 19. The method of claim15, wherein said foreign surface particles are removed and/orredistributed with a pressurized gas stream.
 20. The method of claim 19,wherein said pressurized gas stream is nitrogen or argon at about 25psi.